1. Field of the Invention
The present invention relates to conductive pastes. In particular, it relates to a conductive paste for forming internal electrodes of composite ceramic electronic components such as composite inductors, composite ceramic capacitors and multilayer ceramic substrates.
2. Description of the Related Art
Generally, composite inductors must have low resistance. The internal electrodes thereof are thus composed of a conductive paste composed of, or mainly composed of, silver, i.e., a silver paste.
The silver paste is prepared by mixing a predetermined amount of silver powder and a predetermined amount of an organic vehicle. The organic vehicle is prepared by dissolving ethyl cellulose in a solvent such as terpineol or butyl carbitol.
FIG. 1A is a perspective view of a composite inductor made using the silver paste. FIG. 1B is a perspective exploded view for explaining the structure and the manufacturing method of the composite inductor.
As shown in FIG. 1A, the composite inductor includes an element (element chip) 51 and a pair of external electrodes 53a and 53b. The element 51 includes a multilayer coil 52, and the two ends of the coil 52 are connected to the external electrodes 53a and 53b, respectively.
Referring now to FIG. 1B, the composite inductor is made as follows. First, a plurality of magnetic green sheets 54, each having a coil pattern (internal electrode) 52a printed on a surface using the silver paste by screen-printing or the like, are stacked. Magnetic green sheets (outer layer sheets) 54a without coil patterns are then stacked on the top and on the bottom of the stacked magnetic green sheets 54. The stacked magnetic green sheets 54 and 54a are press-bonded. The coil patterns 52a become connected to each other through via holes 55, thereby forming a coil 52. After the stacked sheets (green element) are baked to prepare the element 51, a conductive paste is applied on the two ends of the element 51 and baked to form the external electrodes 53a and 53b, as shown in FIG. 1A.
In an actual fabrication process, mother sheets, each provided with many coil patterns, are stacked and press-bonded to form a mother composite. The mother composite is cut into chips of a predetermined size. After the chips are baked, external electrodes are formed on the chips. According to this process, a large number of composite inductors can be fabricated simultaneously.
The composite inductors manufactured by this process, however, suffer from delamination of the ceramic layers and undetectable microcracks. In particular, although use of a conductive paste containing a highly dispersible silver powder or silver-containing powder having uniform particle size results in high print quality, structural defects such as delamination or cracks frequently occur. Thus, high reliability is not easily achieved.
The use of these conductive pastes causes structural defects because internal electrodes made therefrom shrink rapidly at around 400xc2x0 C. during the baking due to uniform particle size (sharp particle-size distribution) of the powder. This causes delamination of adjacent ceramic layers sandwiching the internal electrodes, and microcracks. The particular temperature at which rapid shrinkage occurs depends on the particle size of the silver powder, the type of resin constituting the organic vehicle, and the like. Generally, rapid shrinkage occurs in the temperature range of 350 to 500xc2x0 C.
Other types of known ceramic electronic component such as composite ceramic capacitors and multilayer ceramic substrates also have the above-described drawbacks.
An object of the present invention is to overcome the drawbacks of the conventional art by providing a conductive paste that enables efficient production of composite ceramic electronic components free of structural defects such as delamination and cracks.
To achieve this object, the present invention provides a conductive paste for forming an internal electrode of a composite ceramic electronic component, the conductive paste containing (a) a metal powder comprising silver as the main component and (b) an organic vehicle. The metal powder has an average particle diameter of about 1.5 to 3.5 xcexcm. The cumulative volume percent of particles of the metal powder within the average particle diameter xc2x10.5 xcexcm in a particle-size distribution is about 40% or less. The average particle diameter and the cumulative volume percent are determined with a laser diffraction/scattering analyzer.
Use of the above-described metal powder reduces or prevents structural defects, such as delamination and cracks, of the composite ceramic electronic component without significantly degrading printability. Thus, highly reliable composite ceramic components with desired characteristics can be efficiently manufactured.
Since the metal powder containing silver as the main component has a wide particle-size distribution range, rapid shrinkage of the internal electrodes can be moderated or eliminated during baking, thereby achieving gradual shrinking. As a result, the structural defects such as delamination and micro cracks can be efficiently prevented.
More preferably, the average particle diameter of the metal powder is in the range of about 2.0 to 3.0 xcexcm. Moreover, the cumulative volume percent of the particles of the metal powder within the average particle diameter xc2x10.5 xcexcm in a particle-size distribution is more preferably about 36% or less.
Examples of the metal powder containing silver as the main component include a silver-palladium alloy powder and a mixture of silver powder and palladium. Alternatively, a powder of an alloy of silver and a metal other than palladium, or a mixture of silver powder and a metal powder other than palladium powder may be used.
The average particle diameter of the metal powder is in the range of about 1.5 to 3.5 xcexcm. At an average particle diameter exceeding about 3.5 xcexcm, the linearity of the printing is degraded, and a conductive paste containing such a metal powder is not suited for fine line printing. At an average particle diameter of less than about 1.5 xcexcm, the internal electrodes are not sufficiently exposed at the side faces of the composite element prepared by stacking and press-bonding the green sheets, and baking the press-bonded green sheets. As a result, the connection between the internal electrodes and the external electrodes becomes unreliable. The composite ceramic electronic component made using the conductive paste may be a composite inductor.
Composite inductors must have low resistance. Thus, a metal powder mainly containing silver is generally used as the conductive component of the conductive paste for forming internal electrodes of the inductors. When the present invention is applied to making internal electrodes of a composite inductor, the resulting composite inductor is free of structural defects such as delamination and cracks, thereby achieving high production efficiency.